This project will be devoted to studies on age related changes in immunologic and biochemical parameters of lymphoid cell function. The major areas studied will be (1) the role of accessory cells in age related decline in the response to T cell mitogens, (2) the possible age related change in the salvage pathway for thymidine, (3) the capacity of lymphoid cells from young and old animals to increase membrane phospholipid biosynthesis in response to stimulators and (4) the capacity of lymphoid cells from young and aged animals to respond to certain aromatic hydrocarbons by the induction of synthesis of enzymes which metabolize them.